Hard disk drives are used in almost all computer system operations. In fact, most computing systems are not operational without some type of hard disk drive to store the most basic computing information such as the boot operation, the operating system, the applications, and the like. In general, the hard disk drive is a device which may or may not be removable, but without which the computing system will generally not operate.
The basic hard disk drive model includes a storage disk or hard disk that spins at a designed rotational speed. An actuator arm with a suspended slider is utilized to reach out over the disk. The slider includes a head assembly that has a magnetic read/write transducer or head for reading/writing information to or from a location on the disk. The complete head assembly, e.g., the suspension, slider, and head, is called a head gimbal assembly (HGA).
In operation, the hard disk is rotated at a set speed via a spindle motor assembly having a central drive hub. There are tracks at known intervals across the disk. When a request for a read of a specific portion or track is received, the hard disk aligns the head, via the arm, over the specific track location and the head reads the information from the disk. In the same manner, when a request for a write of a specific portion or track is received, the hard disk aligns the head, via the arm, over the specific track location and the head writes the information to the disk.
To read data from a disk, a magnetic head must remain centered on a data track. Typically, this is performed by using position information called servo codes written onto to the disk which is then read by the head. A feedback system uses the servo codes to maintain the position of the head relative to a desired data track. Current industry trends are looking for ways to increase the data density of hard disk drives. One method is to reduce the track pitch and thereby increase the density of tracks on the disk. As a result, precise writing of servo data onto the disk is important.
Previously, dedicated servo-writing devices were used to write the servo data onto the disk. However, due to the reduced size of hard disk drives, and the need for the covers to be in place for proper operation of many drives, a process known as self servo-writing is often used. In the self servo-writing process, the read/write heads of the hard disk drive are used to write the servo data onto the disks. Typically, a position signal is used to servo control the actuator for the read/write head is derived from a closed loop control system which monitors the readback amplitude of patterns (e.g., a position error signal (PES)) that were written in a previous step. The shape of the previously written track is derived based upon the PES using a transfer function, which represents the behavior of the actuator in response to a signal. Once the shape of the track is known, a feed-forward signal is generated which will compensate for errors in the shape of the previously written track. The feed-forward signal controls the position of the actuator when subsequent servo data is written for a track. Optimally, any errors in the shape of the track just written will be eliminated in the subsequent track due to the corrections provided by the feed-forward signal.
Thus, the feed-forward signal is based upon modeling the mechanical behavior of the hard disk drive due to excitations to the control loop. However, there is often some error in measuring the PES and accurately deriving a feed-forward signal which will effectively eliminate track shape errors in subsequent tracks. Thus, a minor distortion in the track shape will magnify in subsequently written tracks if it is not identified and corrected.